In conventional video coding formats, such as the H.264/AVC (Advanced Video Coding) and H.265/HEVC (High Efficiency Video Coding) standards, video frames in a sequence have their size and resolution recorded at the sequence-level in a header. Thus, in order to change frame resolution, a new video sequence must be generated, starting with an intra-coded frame, which carries significantly larger bandwidth costs to transmit than inter-coded frames. Consequently, although it is desirable to adaptively transmit a down-sampled, low resolution video over a network when network bandwidth becomes low, reduced or throttled, it is difficult to realize bandwidth savings while using conventional video coding formats, because the bandwidth costs of adaptively down-sampling offset the bandwidth gains.
Research has been conducted into supporting resolution changing while transmitting inter-coded frames. In the implementation of the AV1 codec, developed by AOM, a new frame type called a switch_frame is provided, which may be transmitted having different resolution than that of previous frames. However, a switch_frame is restricted in its usage, as motion vector coding of a switch_frame cannot reference motion vectors of previous frames. Such references conventionally provide another way to reduce bandwidth costs, so the use of switch_frames still sustains greater bandwidth consumption which offsets bandwidth gains.
In the development of the next-generation video codec specification, VVC/H.266, several new motion prediction coding tools are provided to further support motion vector coding which references previous frames. New techniques are required in order to implement resolution change in a bitstream with regard to these new coding tools.